Sometimes the specific infection and source of sepsis can't be identified.

What causes the symptoms of sepsis?

Usually, your immune system keeps an infection limited to one place. This is known as a localised infection.

Your body produces white blood cells, which travel to the site of the infection to destroy the germs causing infection.

A series of biological processes occur, such as tissue swelling, which helps fight the infection and prevents it spreading. This process is known as inflammation.

If your immune system is weak or an infection is particularly severe, it can quickly spread through the blood into other parts of the body. This causes the immune system to go into overdrive, and the inflammation affects the entire body.

This can cause more problems than the initial infection, as widespread inflammation damages tissue and interferes with blood flow.

The interruption in blood flow leads to a dangerous drop in blood pressure, which stops oxygen reaching your organs and tissues.

People at risk

Everybody is potentially at risk of developing sepsis from minor infections. However, some people are more vulnerable, including people who:

have a medical condition that weakens their immune system – such as HIV or leukaemia

are receiving medical treatment that weakens their immune system – such as chemotherapy or long-term steroids